170 C. E. Beecher — Ventral Integument of Trilobites. 



The last specimen to be noted in this connection is the indi- 

 vidual of Ptychoparia striata, already mentioned as described 

 by Jaekel. 8 A reduced photographic reproduction of his figure 

 (Plate V, fig. 1) is presented here for comparison with similar 

 structures, as described in Triarthrus, Calym,mene, Ceraurus y 

 and Asaphus. From the data here deduced, it would seem 

 obvious that the specimen shows the imprint of the ventral 

 integument in the axial region, the dorsal test and filling of 

 the body cavity having been removed. As in Triarthrus, the 

 body has suffered collapse, thus bringing the dorsal and ventral 

 walls quite near together. In the middle of each of the five 

 or six anterior ventral arches is a groove left by the solution 

 of the chitinous median apodeme, or buttress. On either side 

 are two oblique grooves limiting two subangular areas, and 

 outside of these are two other oblique grooves marking off 

 subrhombic areas. The grooves in each case represent the 

 cavities left by the removal of the chitinous thickenings of the 

 membrane of the trilobite. Jaekel's attempt to remove the 

 rock filling these areas naturally was ineffectual, since the latter 

 represent the actual impression of the ventral integument. 

 Were they simply the fillings of the hollow leg joints, as he 

 claims, they should be readily detached from the matrix. 



The foregoing descriptions and discussions of the character 

 of the ventral integument in trilobites would have little or no 

 scientific value, and would be about as useless as a minute analysis 

 of the nodes and tubercles on the glabella of a P/iacops, were 

 it not for the fact that from them it is possible to reach some 

 conclusions regarding the musculature of trilobites, and thus 

 add something to the knowledge of their internal organization. 



In the abdomen of a normal crustacean, as is well known, 

 there is a pair of longitudinal dorsal muscles, the extensors of 

 the abdomen. They divide into bundles, which are attached 

 on the inner surfaces of the tergites of the somites. Like- 

 wise, on the ventral side, there is a larger pair of longitudinal 

 muscles, the flexors of the abdomen, from which strands are 

 given off and attached to each sternal arch. The strands from 

 one somite unite with the main bundles within the" cavity of 

 the next anterior somite. In a diagrammatic form, this dis- 

 position of the ventral muscles is represented in the accom- 

 panying figure (fig. 1). 



Sow, since in Crustacea it is of very common occurrence to 

 have chitinous extensions of the integument within the body 

 cavity either to divide or to support organs, as well as for the 

 attachment of muscles, it seems a necessary conclusion to refer 

 the thickenings and buttresses on the ventral membrane of 

 trilobites to the same class of structures, which are usually 



